There are eight naturally occurring forms of vitamin E; namely, the alpha, beta, gamma and delta classes of tocopherol and tocotrienol, which are synthesized by plants from homogentisic acid. α and γ -tocopherols are the two major forms of the vitamin, with the relative proportions of these depending on the source.
a-tocopherol is essential, has the highest biological activity and predominates in many species.
The alpha-tocopherol form is the one found in the largest quantities in human blood and tissue. Small amounts of the γ form are also found. In plasma, a-tocopherol is found in all lipoprotein fractions but mostly is associated with apo B-containing lipoproteins. α -Tocopherol is associated with very-low-density lipoprotein when it is secreted from the liver. In the rat, about 90% of total body mass of a-tocopherol is recovered in the liver, skeletal muscle and adipose tissue.
The preferential distribution of alpha-tocopherol in humans over the other forms of tocopherol stems from the faster metabolism of the other forms and from the α -tocopherol transfer protein (α -TTP). It is due to the binding affinity of α -tocopherol with alpha-TTP that most of the absorbed beta-, gamma and delta-tocopherols are secreted into the bile and excreted in the faeces, while α -tocopherol is largely excreted in the urine.
α -tocopherol in human body
